ADPCM (Adaptive Differential Pulse-code Modulation) is a technique for encoding a differential value dn between a sampled value of one sample ago (index: n−1) and a current sampled value (index: n) with respect to a PCM (Pulse Code Modulation) signal of a sampled and digitally-coded sound signal, by using (adapting) a quantization width Δn corresponding to the differential value. With this technique, the PCM signal can be efficiently compressed.
An ADPCM encoding apparatus and an ADPCM decoding apparatus according to a prior art will be described below with reference to FIGS. 1A and 1B. In the ADPCM encoding apparatus shown in FIG. 1A, an inputted analog sound signal is sampled by an A/D converter 1, so that the signal is converted into a digital value Xn. Next, the digital value Xn is encoded using an ADPCM method, and the encoded value Dn is stored in a memory 8.
On the other hand, in the ADPCM decoding apparatus shown in FIG. 1B, the encoded signal dn is read out from the memory 8. Thereafter, the encoded signal Dn is decoded (reproduced) using the ADPCM method, and the decoded value Yn is converted back into the analog sound signal by a D/A converter 13 (see Patent Document 1).
For example, a signed 16-bit PCM code can be converted to a signed 4-bit compressed ADPCM code using the aforesaid ADPCM method.
Next, the operation of the ADPCM encoding apparatus and the ADPCM decoding apparatus respectively shown in FIG. 1A and FIG. 1B will be described in detail.
In the ADPCM encoding apparatus shown in FIG. 1A, first, a differential value dn between a digital value Xn of the sound signal at the current time and a decoded signal Yn-1 of one sample ago obtained through a decoder 5, an adder 6 and a delay device 7 is obtained by an adder (subtractor) 2.dn=Xn−Yn-1 
Thereafter, an encoder 3 converts the differential value dn obtained by the adder 2 into a quantized value Dn (an ADPCM value) by using an adaptive quantization rate Δn (an adaptive quantization characteristic) inputted from an adaptive quantizing section 4. In the quantization process at this time, the differential value dn is divided by the adaptive quantization rate Δn, and the quotient obtained by performing the division calculation is converted into an integer, for example.Δn=Δn-1·M(Dn-1)Dn=[dn/Δn]
Incidentally, the M in the above equation of the adaptive quantization rate Δn is a function with an ADPCM value Dn-1 as a variable, and is determined based on the statistical nature of the signal waveform. One example of such configuration is shown in Non-patent Documents 1 and 2, for example. M<1 when the absolute value of the level of the quantized value is small, and M>1 when the absolute value of the level of the quantized value is large. Further, the [dn/Δn] on the right side of the above equation of the ADPCM value Dn represents a maximum integer which does not exceed dn/Δn.
Further, the ADPCM value Dn calculated by the encoder 3 is stored in the memory 8. By repeatedly performing the above process, the analog input signal (the sound signal) is digitally converted into an ADPCM signal, and the ADPCM signal is stored in the memory.
Incidentally, the decoded signal Yn-1 one sample before the current time obtained through the decoder 5, the adder 6 and the delay device 7 is obtained as below.
First, the ADPCM value Dn-1 of a digital value Xn-1 of the sound signal of one sample ago is decoded in the decoder 5 by using an adaptive quantization rate Δn-1, so as to become a variation qn-1.qn-1=(Dn-1+0.5)·Δn-1 
Thereafter, the variation qn-1 outputted from the decoder 5 and a decoded digital value Yn-2 of further one sample ago outputted from the delay device 7 are added by the adder 6, and thereby the decoded value Yn-1 is calculated.Yn-1=Yn-2+qn-1 
The decoded value Yn-1 obtained in such a manner is delayed by the delay device 7, and the delayed decoded value Yn-1 is inputted to the adder 2. Further, the differential value dn between the delayed decoded value Yn-1 and the digital value Xn of the sound signal at the current time is obtained by the adder 2. In the ADPCM encoding apparatus shown in FIG. 1A, the aforesaid process is repeated, so that the encoding operation using the ADPCM method is performed.
Next, the operation of the ADPCM decoding apparatus (the ADPCM decoder) shown in FIG. 1B will be described below. First, in a decoder 10 of the ADPCM decoding apparatus, the ADPCM value Dn of the digital value Xn of the sound signal read out from the memory 8 is decoded by using the adaptive quantization rate Δn inputted from an adaptive quantizing section 9 to calculate a variation qn. Incidentally, similar to the ADPCM encoding apparatus, the adaptive quantization rate Δn is a function of the ADPCM value Dn-1 of the digital value Xn-1, and is determined based on the statistical nature of the signal waveform.Δn=Δn-1·M(Dn-1)qn=(Dn+0.5)·Δn 
Further, the variation qn calculated by the decoder 10 and the decoded value Yn-1 of one sample ago outputted from a delay device 12 are added by an adder 11 to obtain the decoded value Yn.Yn=Yn-1+qn 
By repeatedly performing the above process, the ADPCM value Dn is read out from the memory 8, and the decoded value Yn corresponding to the ADPCM value Dn is obtained. Further, the obtained decoded value Yn is converted into an analog sound signal by the D/A converter 13, and the sound signal is outputted.
Incidentally, the process operation of the ADPCM encoding apparatus and the ADPCM decoding apparatus is described using an example in which the ADPCM value is inputted and outputted through a memory (storage device or a recording device); however, the ADPCM value may also be, for example, an input/output signal with respect to a transmitted/receiver.